The present invention relates to an interface circuit for interconnecting a time multiplex link and a digital equipment comprising a data transmitter/receiver whicih is at least partially under its own control. The digital equipment may, for example, be a telephone set having a display, or a processor, or a controller for a multiservice subscriber.
The time multiplex link is preferably a two-way link designed to transmit data and/or speech signals that have been conventionally digitized on a byte-by-byte basis.
One such multiplex link under the control of a clock moduIe constitutes, for example, a local communication network for data and/or speech signals between equipments connected thereto. Alternatively, it may be connected to a switching node in a network enabling equipments served by other multiplex links to be reached.
In a known manner, telephone sets are connected to such two-way multiplex links by converting interface circuits of the codec or cofidec type, each of which serves one or more telephone sets both for transmission and for reception.
The analog signals provided by the microphone of a telephone set are converted by the converting interface circuit serving that set into bytes each of which corresponds to one speech sample. These bytes are transmitted one-by-one over the same time slot of a time multiplex link during successive frames under the control of the clock arrangement which governs the link. This clock arrangement provides, in particular, a clock signal which is used by the converter circuit for its own internal operation, together with a synchronization signal which controls byte transmission.
The codec or cofidec type converter circuit serving a telephone set also receives a second synchronization signal from the clock arrangement in order to trigger taking account of bytes which are transmitted to the telephone set in question over a given time slot during successive frames, with the bytes which are taken into account then being converted into signals which can be used by the transducer in the telephone set being served.
Since codec or cofidec converter circuits are standardized for a given type of coding (A-law or U-law) and for a selected basic time multiplex link (24, or 30/32 time slots), it is possible to connect codec or cofidec circuits of the same standard to a corresponding time multiplex link simply by connecting them in parallel, so long as the number of time slots required by these circuits corresponds to the number of time slots available on the link.
However, connecting digital equipment which transmits/receives data to a time ultiplex link conventionally requires the addition of a data interchange interface for taking account of differences in clocks, data rates, and/or formats which exist between the digital equipment and the time multiplex link, in order to make them compatible with each other.
In particular, digital equipment designed to transmit or receive digital data generally includes control logic based on a microprocessor, which is at least partially dedicated to such control. The exchange and/or processing of data by digital equipment often makes use of messages which may be variable in format and/or longer than the single byte which is used as the standard time slot format in two-way time multiplex links of the telephone type.
Consequently, the data interfaces associated with such equipments for connecting them to time multiplex links are generally designed on a case-by-case basis. They are not directly reusable, which makes them expensive and inconvenient insofar as they cannot be mass produced and they lead to an increase in the number of different circuits that need to be stored as spare parts.